New York Times June 16, 1998

The Proof Is in the Neutrino

By SIMON SINGH

This month, a team of physicists working deep inside a Japanese mine shaft announced that the neutrino, perhaps the most mysterious particle in the universe, does indeed have mass. Although fellow experimenters applauded the discovery and the press reported it enthusiastically, the most joyous response came from theoretical physicists who devote much of their lives to conjuring up explanations of the universe and then must wait for experimenters to prove them true.

The neutrino, one of the building blocks of the universe, is the most ghostly of particles, inasmuch as it has evaded almost all methods of detection. Its story began in 1930 when experimenters studying the particles from the decay of radioactive materials were confounded by their flight path. The detected particles did not fly off at random, but were skewed in a certain direction.

To make sense of this and also to retain some sense of harmony and balance, theorists hypothesized that an undetected particle, the so-called neutrino, must be flying off in the opposite direction. It took another 20 years before experimenters were able to prove that the neutrino really did exist and was not just a theoretical convenience.

More recently some theorists began to believe that the neutrino has a minute mass, and over the last decade experimenters have been trying to prove or disprove their hypothesis. However, they can measure a particle only when it interacts with their detector, and neutrinos are notoriously reluctant to interact with anything.

Unlike a photon of light, which will readily interact with the retina and be absorbed by a sprinkling of flimsy cells, a neutrino can pass through six trillion miles of lead without leaving any trace of its passage. It took one of the subtlest measurements in history, made by one of the most sensitive of detectors, to confirm that the neutrino does indeed have mass.

This breakthrough illustrates that the progress of science is a continual to and fro between theorists and experimentalists. While the theorists sit with pencil and paper scribbling models of the universe, it is up to the experimentalists in the laboratory to find a way of testing these theories. Occasionally, the experimentalists lead the way, generating results that force the theorists to revise their models or concoct new ones. This was the case in the 1950's, when physicists discovered new particles (the so-called particle zoo) whose presence had not been predicted by any existing theory.

Such a "paradigm shift" can have a traumatic impact on older theorists, who are left behind while a new generation picks up the pieces. A particularly striking example of this occurred in the early 1900's, when the quantum revolution upended physics, displacing an entire generation.

In recent years, theorists have been in the vanguard, postulating the Standard Model, which has been very successful at explaining experimental results. Since then, theorists have been developing new theories, some of them refinements of the Standard Model, others more radical.

The trouble is, it has been difficult to verify these new theories, because their predicted repercussions could not be tested by any known technology. So, while experimentalists tried to invent better detectors, the theorists were pushing farther ahead, adding more hypotheses to theories that had yet to be proved. This created a house of cards, a beautifully constructed architecture of theories that rested on purely speculative foundations.

The only way to shore up these theories, or demolish them, was by experimentation, the ultimate arbiter of truth. Sir Arthur Eddington, a formidable experimenter in the early 20th century, called experimentation "an incorruptible watch-dog." Max Planck, one of the founders of quantum theory, said, "An experiment is a question which science poses to Nature, and a measurement is the recording of Nature's answer." The challenge is in constructing the right experiment. And the scientists in Japan succeeded in doing this.

Thanks to them, the hypothesis that the neutrino has a mass is now a demonstrable fact. This knowledge affects theories about the engine that powers the sun. It also may explain why astronomers see only a fraction of all the material they expect to find in the universe, and it should help determine the fate of the universe -- will it expand forever or eventually collapse in on itself? All of this depends on the mass of the neutrino.

Theorists suspected the neutrino has a mass. Experimenters looked, and they found it. For most theorists this is a joyous occasion. But there are others whose theories have relied on a neutrino devoid of mass. For them this month's announcement destroys their hypothesis. It will mean erasing what is on the blackboard and starting all over again.